Multi-mode broad-band selective coupler



June 20, 1967 (5. B. SLEEPER, JR 3,327,250

MULTI-MODE BROAD-BAND SELECTIVE COUPLER Filed Nov. 16, 1964 ATT RNEY United States Patent 3,327,250 MULTI-MODE BROAD-BAND SELECTIVE CUUBLER George B. Sleeper, In, Sherburne, N.Y., assignor to Technical Appliance Corporation, Sherburne, N.Y., a corporation of Delaware Filed Nov. 16, 1964, Ser. No. 411,514 3 Claims. (Cl. 333-21) This invention relates to wave energy transmission systems and especially it relates to multi-mode broad-band selective couplers for such systems.

A principal object of the invention is to provide an improved mode discriminating coupler for coupling discrete orthogonally oriented waveguides to another waveguide for simultaneous non-interfering transmissions of orthogonal modes therealong.

Another principal object is to extend the transmission band width of a mode-coupled waveguide system while achieving a minimum of cross-talk between simultaneous but orthogonally related electric fields that are being propagated through the waveguide system.

Another object is to provide an improved mode selecting coupler or a mode mixer or transducer, for coupling two orthogonally related waveguides into a circular or square waveguide for simultaneous discrete wave energy transmission therethrough.

Various forms of so-called mode couplers have been proposedheretofore, for coupling orthogonally related electric fields for simultaneous non-interfering transmission through a common waveguide. A typical such mode coupler is shown in United States Patent No. 2,961,618. As described in that patent, a circular waveguide has a mode selecting junction with a rectangular waveguide. The circular guide is arranged to propagate electro-magnetic wave energy with a selected electric field mode,

while the rectangular guide is arranged to propagate the electro-magnetic energy at the same frequency as the first mode but orthogonally related thereto. In order to achieve the desired simultaneous propagation of both modes with a minimum of cross-talk or interference, the rectangular guide is junctioned to the circular guide while using a tuned iris, and the circular guide is provided with a diametn'c septum so positioned with respect to the said iris that it overlaps that iris, and divides the circular guide junction portion into two parts each of which is beyond the cut-off frequency f of the system. In other words, the septum operates as an inductive element placed in shunt with the iris. The dimensions of the iris and the position of the septum edge are such that the shunt effect of the septum is tuned substantially to the resonant frequency f of the electric field of the wave energy being propagated through the rectangular guide.

However, the prior known arrangement has the disadvantage that the voltage standing wave ratio is more than is desirable for certain uses, and the effective band width of the wave energy that can be transmitted is less than 'is required for certain uses.

I have discovered that by mounting an additional septum in off-set relation with respect to the prior septum, both axially and longitudinally of the circular or square waveguide and with respect to the iris, it is possible to achieve a lower voltage standing wave ratio while preserving the required decibel isolation required to prevent undesired interference between the two modes.

A feature of the invention relates to the novel organization, arrangement, location and dimensioning of parts which cooperate to provide an improved broad-band mode selecting coupler for orthogonally related waveguides.

Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed descriptions, and the appended drawings.

In the drawing,

FIG. 1 is a schematic perspective view of the prior known mode selecting coupler;

FIG. 2 is a top plan view of a wave transmission system embodying the novel mode coupler according to the invention;

FIG. 3 is a right-hand end view of FIG. 2;

FIG. 4, FIG. 5 and FIG. 6 are sectional views of FIG. 2 taken respectively along the lines 4-4, 5-5, and 66 thereof;

FIGS. 7, 8 and 9 are equivalent schematic transmission line representations of the prior known mode coupler such as illustrated in FIG. 1 and used in explaining the advantages of the invention as embodied in FIGS. 2-6;

FIG. 10 is a similar schematic equivalent transmission line representation of the mode coupling system of the invention useful in explaining the advantages over the prior known mode coupler;

FIG. 11 is a representation of the standard Smith Chart showing the charted characteristics with respect to impedance match of the prior known mode coupler and the mode coupler of the invention.

Various systems of microwave communication have been used wherein two discrete wave energy transmissions are propagated without mutual interference, through a waveguide, by using mutually orthogonal orientations of the dominant mode (TE Such systems require a mode coupling junction of special design. A typical such arrangement is schematically shown in FIG. 1 wherein a square or circular waveguide C is coupled to two orthogonal arms A, B. Arm A may be connected to a source of wave energy whose electric field is represented by the arrow E for example in the TE mode; while the arm B is connected to a source of wave energy of the same frequency but with the electric field represented by the arrow E for example in the TE mode. Arm B communicates with the junction through an iris 10, and extending transversely of the side walls of the arm A is a conductive plate or septum 11. The junction between arm B and guide C is in a shunt or magnetic plane junction for the E polarization, that is the narrow transverse dimension of arm B is parallel to the polarization E and the longitudinal axis of arm B is normal to the guide C. Consequently arm B will couple only with the E polarization in guide C, which is parallel with the dominant mode TE in that guide. In the well-known manner the iris 10 has its window edges dimensionally chosen so that the iris size is smaller than the inside cross-sectional dimensions of arm B. The septum 11 is in a plane parallel to the plane of iris 1G and therefore it is in the plane of polarization of E In the well-known manner septum 11 divides the arm A adjacent the junction into two equal portions each of which is beyond the cut-off frequency f of the system. Heretofore the septum 11 has extended beyond one edge of the iris 10 so as to overlap it longitudinally and to such an extent that the shunt combination thereof is tuned substantially to resonance for the frequency of the E polarization. However, it has been found that such a mode selecting coupler, while it does effect the discrimination between the two modes E E for transmission purposes, nevertheless it is accompanied by a relatively high voltage standing wave ratio and a relatively low db return loss. The mode selecting coupler according to the invention and as illustrated in connection with FIGS. 2-6, enables the attainment of a substantially lower voltage standing wave ratio and an impedance match embodying a substantially higher db return loss. In other words, there is achieved the same low VSWR as in prior known couplers but over a much greater band width.

Referring to FIGS. 2P6 there is shown input arm A which may include the rectangular waveguide 12 coupled through an intervening stepped or tapered matching transformer section 13 to a square or round waveguide 14. O-rthogonally connected to waveguide 14 is another rectangular waveguide 15 whose narrow dimension b is parallel to the narrow dimension W of the iris 10 in the wall of guide 14. For the purpose of efiecting unidirectional coupling between the guides, there is provided the septum 11 which may be of metal or high conductivity material extending transversely midway across the guide 14. Septurn 11 is in a plane parallel to the electric field E of the wave energy in guide 15, but is perpendicular to theelectric field E of the wave energy in guide 12. In accordance with standard terminology the width or longer inter-wall dimension of each guide is designated a, a a and the height or shorter inter-wall dimension is designated 12, b b These respective guides are designed in the well-known manner so that the desired wave energy mode vector is parallel to the b dimension. The guides 14 and 15 can be provided with the usual tuning screws or trimmers 16, 17.

As explained hereinabove, the iris 10 is proportioned in dimensions and with respect to the septum 11, so that the shunt effect of the septum is tuned substantially to the resonant frequency f of the electric field of the wave energy being propagated through the guide. The symbol f represents the center frequency of the band of frequencies to be transmitted. Likewise and in accordance with the invention there is provided an additional and highly conductive septum 18 located so that it overlaps the iris 10. The septum 18 is parallel to the E mode in guide .15 and is olfset longitudinally in guide 14 with respect to the septum 11. Thus the forward edge of septum 11 may be located a distance 1' from the said center line; and the forward edge of septum 18 may be located a distance h from that center line. Similarly the septum 18 may be offset axially from the center line of,

guide 14 a distance m, leaving a distance it between septum 18 and the'adjacent wall of guide 14 and also leaving a distance pbetween the septum,18' and the opposite wall of the guide. The width or narrow dimension of septum 11 considered along the longitudinal axis of guide 14 may be k, as may be also the narrow dimension of the septum 18. While the guide 14 may be a square waveguide having a side length of I it may be, if desired, a circular waveguide of diameter 1. For convenience in reference, the following symbols and their meaning are used herein:

Symbols f=frequency f center frequency Af=deviation from f Af/f band width f,,==cut-olf frequency A=wave length X =guide wave length l cut-off wave length Z=guide impedance .a=guide height vb: guide width a =resonant iris height .b =resonant iris width y=normalized admittance m=attenuation Meaning of 1.065/ 1. Thus in one particular dimensional configura tion design for the 4.4 to 5.0 gc. band, the following dimensions expressed in inches were found to be useful:

a=1.872 11:0.255 b=0.872 j 1.307

a =l.550 k=0.450 b =1.550 m=0.775 a :l.872 11:0.387, b =0.872 1:1.163

In the foregoing design, the thickness of the iris 10 and the thickness of the septums 11 and 18 were 0.032.

While the invention is not limited to any theory, the following is a probable explanation for the improved operational characteristics of the mode coupler. In this connection it is desirable first to translate the known mode coupler such as shown in FIG. 1 into equivalent schematic transmission circuit. If one considers the coupler junction as comprised of arms A, B, and C with only the septum 11 and the tuned iris 10, the equivalent circuit is shown in FIG. 7 wherein septum 11 is represented by short circuit stub 19 and wherein the impedance step from square to rectangular waveguide is excluded for practical purposes. Since the square guide or arm C is usually terminated in a load nearly equivalent to its characteristic impedance, it can be replaced .by an equivalent load Y as shown in FIG. 8. To this circuit must be added the transmission line equivalent resonant line of the resonant iris 10, which is represented by the dotted line configuration in FIG. 9 with one end open circuited and one end short circuited and exhibiting a net admittance of Y The admittance as viewed from arm A, namely Y=Y +Y +Y Assuming a ratio of Af/f it can be shown (see T. Moreno, Microwave Transmission Design'Data, Dover Publications, NewYork, 1948, p. 156Fig. 9-19) that Y =0 -j.3 at 4.4 gc. and

The stub admittance Y is determined by the length of the shorted line to the septum 11. This is merely Ag/4 at midband and has lengths of '0.202)\g and O.282)\g at 4.4 and 5.0 gc. respectively. Thus from a standard Smith Chart, we obtain Y -=0 -j.3 at 4.4 gc. and Y =0+j.2 at 5.0 gc. Summing the foregoing, then Y:l.O-j.6 at 4.4 gc. and Y=1.0+j.5 at 5.0 gc.

However, by adding the extra septum 18 hereinabove described, there is created a narrower section of guide, namely 1.16 width by 1.55 height for which =2.32 inches and f =5.1 gc. Since the formulas for cut-01f parameter calculation are approximate, particularly in the region near cut-off, it is possible that this section of waveguide is operating in the high attenuation region just slightly above cut-oh. On that assumption the circuit of FIG. 9 can be modified to represent the stub as lossy, as represented in FIG. 10. It is possible that this value of attenuation could be as much as 10 db for which he return loss would be 20 db and Y would have the following values: Y =.85-j.1 at 4.4 gc. and Y -=.83+j.07 at 5.0 gc. Again summing the above, we

Obtain Y=1.85 '.4 and Y:1.83+j.37. When the values of the foregoing Y are plotted on a well-known standard Smith Chart, respectively with and without the additional septum 18, results are indicated in FIG. 11 from which it will be seen that the admittance group is smaller when the additional septum 18 is present, with a resultant better match over a given bandwidth, or it will have greater bandwidth for a given match level. The significance of the two curves Y and Y is that Y when appropriately provided with transformation matching which moves it to the center of the chart coincident with the larger curve will extend over a lesser range of susceptance values, or for a given bandwidth. it will more nearly match the normalized admittance Y This, in effect, is the achievement of a lower VSWR.

Regardless of the theory above advanced, it is a known fact that the presence of the additional septum 18 does improve the performance of the mode selector junction and is capable of effecting an improved performance over the known junction such as shown in FIG. 1.

While certain specific dimensions, frequencies and other parameters have been listed herein, it will be understood that the same is done merely for purposes of explanation and not by way of limitation on the scope of the invention.

What is claimed is:

1. A selective mode transducer for the transmission of electromagnetic wave energy, comprising a pair of first and second waveguide arms, means coupling said arms to form a common junction through which are to be selectively propagated without mutual interference electric fields of respective transverse polarizations, and means to elfect said coupling including a resonant iris and a transverse septum both of which are substantially parallel to the polarization in the second arm and substantially perpendicular to the polarization in the first arm, and an additional septum parallel to the first septum but ofiset relatively thereto to increase the effective bandwidth of said transmission, the first septum being located midway across the first arm and the second septum being located between the first septum and adjacent the wall of the first arm.

2. The selective mode transducer according to claim 13 in which only the second septum overlaps said iris.

3. A mode transducer for the transmission of electromagnetic wave energy, comprising first means for propagating first and second electric wave energies with their respective electric fields cross-polarized, second means for propagating electromagnetic wave energy with a third electric field polarized parallel to the polarization of one of said fields in said first means, said first and second means forming a junction into which all said fields are coupled, conductive septum means arranged at said junction substantially parallel to the polarization plane of the second and third fields and substantially perpendicular to the polarization of said first fields, and separate parallel, conductive septum means offset with relation to the first septum means to increase the bandwidth of said transmission while minimizing interference between said fields through said junction, said second means being coupled to said junction through a resonant iris, said first means being a waveguide section having a waveguide having dimensions for selectively propagating said energy representing a signal with one polarization and for propagating said energy representing another signal with a polarization transverse to said one polarization, and said first septum means being located midway of the said section, and said separate septum means being located substantially parallel to the first septum means between the first septum means and the adjacent wall of said section remote from said iris.

References Cited UNITED STATES PATENTS 2,961,618 11/1960 Ohm 333-9 FOREIGN PATENTS 767,518 2/1957 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

M. NUSSBAUM, Assistant Examiner. 

1. A SELECTIVE MODE TRANSDUCER FOR THE TRANSMISSION OF ELECTROMAGNETIC WAVE ENERGY, COMPRISING A PAIR OF FIRST AND SECOND WAVEGUIDE ARMS, MEANS COUPLING SAID ARMS TO FORM A COMMON JUNCTION THROUGH WHICH ARE TO BE SELECTIVELY PROPAGATED WITHOUT MUTUAL INTERFERENCE ELECTRIC FIELDS OF RESPECTIVE TRANSVERSE POLARIZATIONS, AND MEANS TO EFFECT SAID COUPLING INCLUDING A RESONANT IRIS AND A TRANSVERSE SEPTUM BOTH OF WHICH ARE SUBSTANTIALLY PARALLEL TO THE POLARIZATION IN THE SECOND ARM AND SUBSTANTIALLY PERPENDICULAR TO THE POLARIZATION IN THE FIRST ARM, AND AN ADDITIONAL SEPTUM PARALLEL TO THE FIRST SEPTUM BUT OFFSET RELATIVELY THERETO TO INCREASE THE EFFECTIVE BANDWIDTH OF SAID TRANSMISSION, THE FIRST SEPTUM BEING LOCATED MIDWAY ACROSS THE FIRST ARM AND THE SECOND SEPTUM BEING LOCATED BETWEEN THE FIRST SEPTUM AND ADJACENT THE WALL OF THE FIRST ARM. 